The present invention relates to electroluminescent devices, which may be used as colored display elements in indicator devices or flat plate multicolored displays.
Electroluminescent elements can be categorized as natural or injection type. The present invention relates to injection type electroluminescent devices which operate by the application of a current through the system, creating luminescence at a p-n junction.
Electroluminescene in organic crystals has been investigated by a number of researchers for practical electroluminescent devices [Materials Science, 7, 43 (1981)]. The primary reason is that a large number of organic materials are known to have extremely high fluorescence quantum efficiencies in the visible spectrum [Topics in Applied Physics: Dye Lasers, (Springer, N.Y., 1977)]. An anthracene single crystal, for example, has been shown to exhibit intense electroluminescence [Physical Review Letter, 14, 229 (1965)]. From a practical point of view, however, electroluminescent devices using organic single crystals are less attractive, since drive voltages of 100 Volts or more are necessary for bright emission. Therefore, studies of electroluminescence in organic materials have concentrated on thin film devices.
The electroluminescent element, in the form of a thin film, is placed between two electrodes. By applying a current across the electrodes, electrical energy will be converted directly to light. The electroluminescent element can operate in a wide range of ac or dc frequency and at low voltages with a high rate of conversion of electric energy to light energy. Injection type electroluminescent devices have mainly been constructed with inorganic semiconductors, such as GaP. Tang and VanSlyke recently reported an injection type luminescent diode constructed of two membrane layers, one p-type organic compound and one n-type organic compound [Applied Physics Letters, 51 (12), 913(1987)]. The device consisted of a double layer structure with a hole transporting diamine layer and a luminescent aluminum complex. Tang et al also reported that doping the aluminum complex with organic luminescent dyes yields luminescence from the dopant materials with higher quantum efficiency [Journal of Applied Physics, 65, 3610 (1989)].
Similar results were obtained by Adachi et al [Japanese Journal of Applied Physics, 27, L269 (1988) and Japanese Journal of Applied Physics, 27, L713 (1988)]. Their devices consisted of three organic layers, a diamine layer for hole injection, a luminescent organic dye layer, and an electron injection layer. By using appropriate luminescent dyes, they obtained desired luminescent colors such as blue, green and red. Their latest study showed that luminescent materials with hole transporting properties exhibit efficient electroluminescence without a hole transporting diamine layer [Applied Physics Letters, 55, 1489 (1989)].
In all these prior art examples, the broad nature of the luminescence spectra of organic dyes result in luminescent colors which are dull and, therefore, not best suited for actual display applications. The emission spectra generally have spectral band widths of 100 to 200 nm. Filters must therefore be used in order to obtain monochromatic light.
It is desirable to obtain bright and efficient electroluminescent devices with narrow luminescent spectral band width.
It is further desirable to obtain electroluminescent devices which can operate at high efficiency and at low voltages.
It is specifically desirable to obtain electroluminescent devices which can be mass produced at low cost and in large areas.
These objects are accomplished by the present invention wherein the luminescence element consists of a thin film of an organometallic compound of the lanthanide series is interposed between two electrodes, one of which is transparent to visible light.
The present invention covers a class of electroluminescent materials with exceptionally narrow spectra emission bands which provide bright colors when incorporated in an electroluminescent device. Further improvement in the design is achieved by interposing a hole conducting layer between the luminescent material and the transparent electrode. A key aspect of the invention is to use as electroluminescent layer organometallic complexes with rare-earth elements of the lanthanide series.
One aspect of the invention is to provide an electroluminescent thin film consisting of an organometallic complex of rare-earth elements of the lanthanide series sandwiched between a transparent electrode of high work function and a second electrode of low work function. In another aspect of the invention, a hole conducting layer is interposed between the electroluminescent layer and the transparent, high work function electrode. In yet another aspect of the invention, an electron conducting layer is interposed between the electroluminescent layer and the electron injecting, low work function electrode.
An object of the invention is to provide an electroluminescent device with narrow band emission spectrum to provide bright emission in the visible region of the spectrum upon the application of a voltage.
A further object of the invention is to provide an electroluminescent device of the above described type in which the luminescent element is sandwiched between a transparent electrode of high work function and an electrode of low work function.
A further object of the invention is to provide an electroluminescent device of the above type which operates at low voltages.
A further object of the invention is to provide a transparent hole conducting layer interposed between the electroluminescent element and the transparent, high work function electrode.
A further object of the invention is to provide an electron conducting layer interposed between the electroluminescent element and the low work function electrode.
A further object of the invention is to provide an electroluminescent device wherein the electroluminescent element, the hole conducting element, the electron conducting element and the electrodes are in the form of thin films which can be fabricated to cover large areas.